Electrical signaling system



April 20, 1937. P. N. ROSEBY 7 ELECTRICAL SIGNALING SYSTEM Filed June 29, 1954 INVENTOR- P 04 1/ NORTON F0555)" Patented Apr. 20, 1937 UNITED STATES PATENT OFFICE ELECTRICAL SIGNALING SYSTEM ware . Application June 29, 1934, Serial No. 732,990

In Great Britain August 3, 1933 2 Claims.

The present invention relates to electrical signaling systems and is more particularly concerned with systems in which telegraphy and telephony are enabled to take place simultaneously over the 5 same circuit. The invention moreover is particularly applicable though not limited to cases in which the signaling is effected over a single line with earth return, which line may be of a temporary nature as in military operations or the like.

One of the, most serious difiiculties which have been experienced in the design of systems of the character indicated is that of providing means for calling on the telephone circuit which shall not interfere seriously with the telegraph equipment.

This equipment will normally be extremely sensitive and is likely to be damaged, or at any rate put out of adjustment, if it is traversed by comparatively high voltage alternating current such as is usually employed for ringing purposes in a telephone circuit. The use of filters which will cut ofi all currents above a comparatively low frequency is not by itself sufiicient since it does not take care of transients and also of the fact that if a magneto generator is used and is permanently connected up the currents generated initially while the crank handle is being turned slowly will be of very low frequency and will not be satisfactorily cut off by the filter.

According to the invention in a signaling system of the type described, damage to the telegraph receiving equipment by telephone calling current is prevented by the use of a filter with a low cut-01f in the telegraph circuit in conjunction with a small condenser in series with the calling generator. As will be readily appreciated, the effect of the condenser is to offer a high resistance to currents of low frequency and since these currents will also be of low voltage, the actual current flowing through the telegraph equipment is not sufiicient to cause interference or damage. As the operator rotates the handle of the generator at a faster speed, both the voltage and the frequency of the current increase and consequently more current passes through the condenser. By the time this current reaches serious proportions, however, the frequency will be such v end of the line and it will be assumed that the apparatus at the remote end is similar. The in-' coming line I has connected to it by way of the up and also the springs 9 which serve to com-' plete a circuit for the transmitter II] from the battery ill by way of the primary of the speech transformer l2. The secondary circuit of this transformer includes the telephone receiver l3 while the circuit over the contact engaged by the switch-hook spring 8 includes the calling signal Hi.

In the arrangement shown this comprises a telephone receiver suitably mounted and provided with an amplifying horn so that it will produce a magnified howl varying in pitch according to the speed of the generator at the other end of the line. Alternatively, the equipment for responding to calling signals in the telephone circuit may consist of a drop indicator of the usual type preferably shunted by a dry rectifier in order that it may operate reliably with the alternating current. This drop could be arranged to close a local circuit for a hell or the like in the usual manner.

The generator 5 is preferably of the type commonly used for telephone purposes which when operated at full speed will produce current of a frequency in the neighbourhood of 150 cycles. The filter 2 is conveniently arranged to pass only currents of a frequency less than about cycles.

It is not essential that the condenser 4 should be normally short-circuited and it might be included in the speaking circuit in so far as it did not interfere with speech requirements. Alternatively it might be included in the lead by which the generator is connected to earth, the important point being that it should be in the circuit to line when the generator is being used. Similarly the generator could merely be disconnected rather than short-circuited When it was not being used.

The equipment will normally be arranged for duplex working and the equipment at the two ends may be identical, or there may be differences due to difierent conditions. The arrangement is Particularly suitable for field signaling purposes as used in military operations, in which case the line may be terminated at one end in buildings of a more or less permanent nature, while the other end may be in the field and'actually on the move.

With the arrangement according to the invention in adidtion to the advantage that independent telegraphy and telephony may take place over the same wire there is the further advantage that the telegraph and telephone positions need not be adjacent since the telegraph and telephone circuits are quite separate, and even calling on the telephone circuit which is likely to produce considerable more disturbance than speech will not interfere with the normal telegraph operation.

I claim:

1. In a combined telephone and telegraph line, a pair of branch circuits connected to the line in multiple, one branch circuit including telegraph equipment and a low pass filter connected in series in said one branch, the other branch including telephone equipment, a calling generator and a condenser connected in series with the telephone equipment in said other branch, said low pass filter preventing signaling current from 5 the generator from passing into the first branch, and said condenser preventing sub-normal frequencies in the telephone circuit from reaching the line.

2. A combined telephone and telegraph circuit 10 branch including in series therein, telephone 15 equipment, a magneto generator, and a condenser, said generator and condenser normally short circuited, and means for removing said short circuit whenever the generator is operated,

said low pass filter and condenser preventing in- 20 terferenoe between said branches.

PHILIP NORTON ROSEBY. 

